This invention relates to improvements in methods of decontaminating water. More particularly, the invention relates to methods of disinfecting water using silver and ultraviolet (UV) radiation.
Over the centuries numerous methods have been used to purify water and render it potable. Typically, this involves removal of particulate matter and disinfection of the water. Although sometimes relying on chemical treatment to disinfect pathogens, some small scale water purification systems, such as those used by backpackers, have recently used filters to remove both particulate matter and most biological and chemical contaminants. However, the use of filters to remove biological contaminants has been largely impractical on the scale of a municipal water treatment facility. For that reason, for over a century many large scale water purification systems have used chemical means to remove non-particulate contaminants. Such chemical purification means have sometimes proved to be somewhat ineffective and aesthetically objectionable when applied to large scale purification systems, such as municipal water purification systems.
Chief among the chemical agents used for decontamination of most biological and chemical contaminants has been chlorine. It is readily available and is generally effective for its intended purpose when properly used. However, many consumers have objected to both the odor and taste that chlorine imparts to drinking water when used even within prescribed limits. Chlorine has also been linked to the formation of carcinogens when water containing organic matter is treated. In recent years, chloramine has replaced chlorine at a number of water treatment plants, at least in part because it has a somewhat less objectionable odor and taste than is typically found in water treated with chlorine. However, over the last few decades, because of aesthetic reasons and shortcomings in adequately treating water supplies in several large metropolitan areas in the United States which has resulted in a number of people becoming sick due to pathogens in the improperly processed water, many consumers have installed water treatment devices in their homes or have taken to consuming bottled water. Accordingly, the costs incurred by the public either indirectly in water treatment at the municipal level or directly in obtaining more reliable or less offensive drinking water can be significant.
It has been known since at least 1000 BC that water kept in silver vessels could be made potable. The bactericidal effects of silver have been known since the mid 1800s. Given contact times on the order of hours, silver ions, Ag+, have been shown to be an effective disinfectant against coliforms and viruses. In water, at concentrations sufficient for bactericidal activity, silver ions do not impart taste, color, or odor and have no apparent detrimental effects on mammalian cells. The only known negative health effect is argyria, an irreversible darkening of the skin and mucous membranes, which is caused by prolonged silver therapy.
The mechanisms of the bactericidal activity of silver have been attributed to reactions between silver and both thiol groups and amino acids and the binding of silver to key functional groups in enzymes. Silver has also been shown to inhibit the respiratory chain and inhibit phosphate uptake in Escherichia coli. 
Silver is not typically used as a large-scale disinfectant in most developed nations, although it is popular in parts of Europe and in some developing nations as a drinking-water disinfectant. It is commonly used to prevent microbial growth in point of use filters, as a co-disinfectant for swimming pool water, which allows for lower chlorine levels in pools, and as a co-disinfectant in hospital hot water systems.
For point of use applications, silver has been impregnated in activated carbon or ceramic filter candles. The desorption of silver from these water purification devices, automatically provides a residual disinfectant effect for users who may not have access to other disinfectants or understand the concept of disinfection and how to determine or measure a safe quantity of disinfectant for potable water. In addition, silver may be useful in potable water storage systems that have long contact times such as cisterns and water storage tanks used in the field by the military.
Two of the principal drawbacks associated with the use of silver as a disinfectant are the need for long contact times and the existence of silver-resistant organisms. Moreover, several reports on the use of silver impregnated water treatment devices suggest that silver provides little benefit as a disinfectant over the life of a typical point of use granular activated carbon (GAC) filter or filter candle. The causes for the reported inability of silver to effectively disinfect water in point of use applications remains largely unreported although silver-resistant bacteria may have been implicated in some cases.
Unlike silver, ultraviolet (UV) radiation is considered a viable process for disinfecting drinking water and wastewater in large-scale water treatment systems because it is an effective means of inactivating pathogens, including bacteria, viruses and protozoa, such as Cryptosporidium parvum and Giardia lamblia, and it does not create significant disinfection byproducts. As with any disinfection process, an important consideration associated with UV radiation is cost. Power requirements for UV systems are primarily a function of the desired fluence (the product of irradiance and exposure time). In addition to an increase in operating costs, an increase in fluence can also result in a significant increase in capital costs. Microbial inactivation goals, which are a function of a target organism, set the UV design fluence which, for water treatment, can typically vary between 40 mJ/cm2 and 140 mJ/cm2. Fluences as high as 170 mJ/cm2 have been reported for 4-Log (99.99%) inactivation of adenoviruses in tertiary-treated wastewater, which indicates that fluences sufficient for inactivation of coliforms (e.g. ca. 8 mJ/cm2 for E. coli) may not provide suitable inactivation of human adenoviruses. Because viruses are reported to be emerging as the pathogens that are most resistant to UV disinfection, they are likely to control the fluence requirements of disinfection processes in many cases. A reduction in the UV design fluence and subsequent capital and operating costs would make UV disinfection more appealing to municipalities who may wish to eliminate disinfection byproducts and improve inactivation of pathogens such as protozoa.
Accordingly, it is an object of the invention to provide a method for improving the disinfection of aqueous systems by inactivation of pathogens, such as bacteria and viruses. It is a further object of the invention to provide an improved method of disinfecting drinking water treatment systems by inactivation of pathogens, such as bacteria and viruses, which will impart no perceptible odor or taste to the water. It is another object of the invention to reduce capital and operating costs of water treatment methods and systems used to inactivate pathogens that employ UV radiation. Additionally, it is an object of the invention to provide an improved method for the disinfection of aqueous systems by inactivation of pathogens, such as bacteria and viruses, which minimizes or eliminates the formation of carcinogens. It is yet a further object of the invention to provide a method for improving the disinfection of aqueous systems by inactivation of pathogens, such as bacteria and viruses, with UV radiation in the presence of silver ion (Ag+).